This invention relates to a hydrometallurgical process for recovering metal values from sulfidic minerals and more particularly to a leaching process wherein a nitrogen dioxide containing gas is passed through an acidic leaching liquor to oxidize the minerals in the leach liquor.
As a preliminary step to winning copper, silver, nickel, cobalt, molybdenum, iron, and/or zinc metal from sulfidic minerals, it is necessary to oxidize the minerals to enable the aqueous solution to solubilize these metal values. The oxidation reactions may be effected by high temperatures roasts or by hydrometallurgical processes involving the use of an aqueous acidic leach liquor containing an oxidant such as nitric acid. The former procedure has the advantages of being relatively non-selective in terms of mineral reactivities and of consuming significantly less energy during the oxidation process as compared with hydrometallurgical techniques. Unfortunately, conventional roast reactors produce large volumes of dilute sulfur dioxide gas which necessitates the use of expensive ancillary equipment for removing sulfur dioxide from the effluents so that environmental protection standards can be met. The hydrometallurgical processes, on the other hand, typically involve relatively expensive reagents such as nitric acid which, from a practical point of view, must be regenerated and recycled. The cost of the equipment needed to regenerate nitric acid in a conventional nitric acid leach liquor represents a significant capital investment. Accordingly, the use of nitric acid for leaching metal values from sulfidic minerals has not had wide commercial success.
In prior application Ser. No. 855,983 entitled "Copper Leaching Process Employing Nitrogen Dioxide," the teachings of which are incorporated herein by reference, a process is disclosed which utilizes nitrogen dioxide to oxidize sulfidic ores to enable metal values to be leached in an aqueous leach liquor. This process has many advantages which include low volumes of off-gases produced, rapid adaptation to changes in mineralogy and grade of the concentrate feed, and the ability to attack even the most refractory minerals. In the process of that application, a particulate sulfidic mineral is added to an aqueous acidic leach liquor and a nitrogen dioxide containing gas is sparged through the resulting slurry. In this single stage leach, the sulfidic minerals are oxidized and the metal values are dissolved. Although this process has many advantages, it does not offer the stoichiometric control that results from the present invention.